Adherence of bacteria to saliva-coated surfaces, saliva-induced aggregation of bacteria and highly specific interactions between different species of oral bacteria (coaggregation) are important determinants in the microbial ecology of the oral cavity. Our studies which assessed the interaction of human parotid salivary proteins with hydroxyapatite surfaces and bacterial cell surfaces support roles for different proteins, tentatively identified as proline rich proteins, in adherence (Mr 22k) and in aggregation (Mr 35k) of Streptococcus sanguis. Isolation of these proteins from hydroxyapatite (HA) and from bacteria after treatment with parotid saliva and their further purification are being carried out for physico-chemical characterization and to quantitatively evaluate their importance in bacteria-surface and bacteria- bacteria interactions. Further studies with rat parotid saliva showed that the Mr 40K protein which selectively absorbed to HA and was associated with the adherence of streptococcal and actinomyces cells to this mineral was made radioactive after in vivo incorporation of C-14-proline. C-14-labeled Mr 40k protein neither aggregated or bound to streptococcal cells indicating that the bacteria-protein interaction occurred only when the protein was attached to the HA. A purified preparation of a high molecular weight proline-rich glycoprotein (PRG) of rat parotid saliva, which did not bind to HA, aggregated Actinomyces viscosus cells. Monoclonal antibody specific for PRG is being prepared to further study this aggregation and to obtain pure preparation of this glycoprotein. Coaggregation between S. sanguis and Propionibacterium acnes on a saliva-coated HA surface occurred, but at a reduced extent in human whole saliva when compared to that determined in buffer. Results of experiments which examined pH effects, hydrophobic effects and salt effects suggest that the higher ionic strength of saliva was, in part, responsible for this reduced coaggregation.